1. Field of the Invention
This invention relates to an edge finding method wherein, from an image signal made up of a series of image signal components representing a radiation image of an object, judgments are made as to whether predetermined picture elements in the radiation image belong or do not belong to part corresponding to an edge in the radiation image. This invention also relates to an apparatus for carrying out the edge finding method.
2. Description of the Prior Art
Techniques for reading out a recorded radiation image in order to obtain an image signal, carrying out appropriate image processing on the image signal, and then reproducing a visible image by use of the processed image signal have heretofore been known in various fields. For example, as disclosed in Japanese Patent Publication No. 61(1986)-5193, an X-ray image is recorded on an X-ray film having a small gamma value chosen according to the type of image processing to be carried out, the X-ray image is read out from the X-ray film and converted into an electric signal (image signal), and the image signal is processed and then used for reproducing the X-ray image as a visible image on a copy photograph, or the like. In this manner, a visible image having good image quality with high contrast, high sharpness, high graininess, or the like can be reproduced.
Also, when certain kinds of phosphors are exposed to radiation such as X-rays, .alpha.-rays, .beta.-rays, .gamma.rays, cathode rays or ultraviolet rays, they store part of the energy of the radiation. Then, when the phosphor which has been exposed to the radiation is exposed to stimulating rays such as visible light, light is emitted by the phosphor in proportion to the amount of energy stored thereon during its exposure to the radiation. A phosphor exhibiting such properties is referred to as a stimulable phosphor.
As disclosed in U.S. Pat. Nos. 4,258,264, 4,276,473, 4,315,318, 4,387,428, and Japanese Unexamined Patent Publication No. 56(1981)-11395, it has been proposed to use stimulable phosphors in radiation image recording and reproducing systems. Specifically, a sheet provided with a layer of the stimulable phosphor (hereinafter referred to as a stimulable phosphor sheet) is first exposed to radiation which has passed through an object, such as the human body. A radiation image of the object is thereby stored on the stimulable phosphor sheet. The stimulable phosphor sheet is then scanned with stimulating rays, such as a laser beam, which cause it to emit light in proportion to the amount of energy stored thereon during its exposure to the radiation. The light emitted by the stimulable phosphor sheet, upon stimulation thereof, is photoelectrically detected and converted into an electric image signal. The image signal is then used during the reproduction of the radiation image of the object as a visible image on a recording material such as photographic film, on a display device such as a cathode ray tube (CRT) display device, or the like.
Radiation image recording and reproducing systems which use stimulable phosphor sheets are advantageous over conventional radiography using silver halide photographic materials, in that images can be recorded even when the energy intensity of the radiation to which the stimulable phosphor sheet is exposed varies over a wide range. More specifically, since the amount of light which the stimulable phosphor sheet emits when being stimulated varies over a wide range and is proportional to the amount of energy stored thereon during its exposure to the radiation, it is possible to obtain an image having a desirable density regardless of the energy intensity of the radiation to which the stimulable phosphor sheet was exposed. In order to obtain the desired image density, an appropriate read-out gain is set when the emitted light is being detected and converted into an electric signal to be used in the reproduction of a visible image on a recording material, such as photographic film, or on a display device, such as a CRT display device.
Recently, in the radiation image recording and reproducing systems which use recording media, such as X-ray film or stimulable phosphor sheets, particularly in such radiation image recording and reproducing systems designed to facilitate medical diagnoses, edges in images are detected from the image signals representing the images. The term "edge" as used herein means part of an image at which the image density (i.e. the value of the image signal) changes sharply. Whether part of an image is to be or is not to be detected as an edge when how much the image density changes at said part may be determined in accordance with the characteristics of the edge which is to be found.
For example, in the course of recording a radiation image, it is often desirable for portions of the object not related to a diagnosis, or the like, to be prevented from being exposed to radiation. Further, when the object portions not related to a diagnosis, or the like, are exposed to radiation, the radiation is scattered by such portions to the portion that is related to a diagnosis, or the like, and the image quality is adversely affected by the scattered radiation. Therefore, when a radiation image is recorded on the recording medium, an irradiation field stop is often used to limit the irradiation field to an area smaller than the overall recording region of the recording medium so that radiation is irradiated only to that portion of the object, which is to be viewed, and part of the recording medium. In such cases, the shape and location of the irradiation field should be found in order that, as disclosed in, for example, U.S. Pat. No. 4,527,060, image information recorded in the region inside of the irradiation field may be read out under appropriate readout conditions, and/or in order that the image signal representing image information recorded in the region inside of the irradiation field may be processed under appropriate image processing conditions. As proposed in, for example, U.S. Pat. No. 4,967,079, in the course of finding the shape and location of the irradiation field, the edge of the irradiation field is detected by utilizing the difference between the image density in the region inside of the irradiation field and the image density in the region outside of the irradiation field (i.e. the difference between the value of the image signal representing image information recorded in the region inside of the irradiation field and the value of the image signal representing image information recorded in the region outside of the irradiation field).
Also, for example, after an image signal representing an X-ray image of a human body, which image is primarily composed of bone patterns and soft tissue patterns, is detected, it is often desired that the image signal components representing the bone patterns and the image signal components representing each of the soft tissue patterns can be processed under different appropriate image processing conditions. Additionally, in the course of finding specific patterns (e.g. tumor patterns) from an X-ray image having a complicated configuration, it is often desirable for different pattern finding methods to be employed for different regions of the X-ray image. In such cases, it is necessary that boundaries between the bone patterns and the soft tissue patterns and between the plurality of the soft tissue patterns can be found. In the course of finding such boundaries, techniques for detecting edges are often utilized.
In general, differentiation operators are utilized to detect edges in images from image signals representing the images. Such techniques are described in, for example, "Gazo-shori no Kihon-giho" (Basic Techniques for Image Processing), Guide to Techniques Edition, p. 35, by Jun-ichi Hasegawa, Yamato Koshimizu, Akira Nakayama, and Shigeki Yokoi, Gijutsu Hyoron Sha. With the techniques utilizing the differentiation operators, differentiation processing is carried out on image signal components corresponding to positions located along a predetermined direction on an image. Thereafter, a peak value of the absolute values of the differentiated values resulting from the differentiation processing is found.
However, in general, radiation images include very much high-frequency noise caused to occur by, for example, sway of radiation, which is employed during the recording of the radiation images. Therefore, the problems occur in that edges in images cannot be detected accurately when the differentiation operators are employed.
FIGS. 5A, 5B, and 5C are explanatory views showing how problems occur when a differentiation operator is utilized during the detection of an edge in an image.
With reference to FIG. 5A, regions 1, 1, each of which is sandwiched between two, approximately parallel curves, have lower image densities than the surrounding regions (i.e. the values of the image signal components representing the image information recorded in the regions 1, 1 are smaller than the values of the image signal components representing the image information recorded in the surrounding regions). FIG. 5B is a graph showing the levels of the image densities (i.e. the values of the image signal components) corresponding to picture elements in the radiation image, which picture elements are arrayed along the chained line, y, in FIG. 5A. As illustrated in FIG. 5B, the image signal components include very much high-frequency noise. In such cases, if a differentiation operator is merely employed, the problems will occur in that the edges (in this example, the boundaries between the regions 1, 1 and the region 2) cannot be detected accurately. In order for the high-frequency noise to be reduced, the image signal components may be smoothed and then processed with a differentiation operator. However, as shown in FIG. 5C, if the image signal components are smoothed, the edge parts will also be smoothed, and therefore the locations of the edges will become unclear. If the image signal components, which have been smoothed, are processed with a differentiation operator, the problems will occur in that the edges are detected at positions deviated from their correct positions.
As another example of the techniques for detecting an edge in an image, the so-called model fitting technique is known. With the model fitting technique, an appropriate function is determined such that the values of image densities (i.e. the values of the image signal components) can be expressed as the values of the function. An edge in an image is found from the inclination of the function, or the like. Such a model fitting technique is described in, for example, the aforesaid publication "Gazo-shori no Kihon-giho", p. 37. With the model fitting technique, the problems occurs in that edges are detected at positions deviated from their correct positions.